Replace comma with semicolon in trace/simple.c

In the master branch in trace/simple.c in writeout_thread (https://github.com/qemu/qemu/blob/master/trace/simple.c#L174) we currently have:
  dropped.rec.length = sizeof(TraceRecord) + sizeof(uint64_t),
  dropped.rec.pid = trace_pid;

It seems to me like a typo that the first line ends with a comma.
Currently this causes no harm, but I think this should be fixed.

It's perfect valid C to terminate a statement with "," instead of ";" - it just has a different meaning. Consider this:

#include <stdio.h>

int main()
{
    if (0)
        printf("Hello!\n"),
        
    printf("Good bye!\n");

    return 0;
}

At a first glance, you'd expect this program to print "Good bye!" - but it does not. Actually, the "," is used here to put the two printf statements into the same block, so this program is the same as:

    if (0) {
        printf("Hello!\n");
        printf("Good bye!\n");
    }

Thus, there is no real bug in simple.c here, but of course it would be better style to clean this up and use ";" instead.

By the way, two lines earlier there is another line ending in ",":

    dropped.rec.event = DROPPED_EVENT_ID,

Fixed in commit 7ff5920717d413d8b7c3ba13d9, which will be in the upcoming 4.0 release.